A sample analyzer which measures, for example, the size of platelets in blood by the flow cytometry method using a laser diode is disclosed, for example, in U.S. Pat. No. 6,525,807. Such analyzers must reduce the noise of the laser diode in order to perform accurate measurements.
Laser diodes are single mode (oscillation at a single wavelength) when light is continuously emitted. However, the oscillation wavelength of the laser diode changing to another wavelength during single mode oscillation (referred to as a “mode hop” below) can be accomplished by changing the magnitude of the current which is supplied to the laser diode, and changing the temperature of the laser diode. When generating a mode hop, a change occurs in the amount of light output from the laser diode, and the change in the amount of light can be detected as noise. This content is disclosed, for example, in Japanese Laid-Open Patent No. H9-178645, and U.S. Pat. No. 5,946,333. This noise is difficult to distinguish from a detection signal detected when measuring very small particles. It is therefore necessary to suppress the generation of noise caused by mode hopping in order to perform measurements with satisfactory precision.
In the art disclosed in Japanese Laid-Open Patent No. H9-178645 and U.S. Pat. No. 5,946,333, methods are used for maintaining the laser diode in a multi-mode oscillation state (that is, a state in which oscillation occurs at a plurality of wavelengths) by applying a drive current obtained by applying high frequency modulation on a base direct current, so that the laser diode does not settle in a single-mode oscillation state. When such a method is used, the lower limit value (the trough part of a sine wave waveform) of the drive current subjected to high frequency modulation can be reduced to less than the threshold current value of the laser output of the laser diode by manually adjusting the amplitude of the high frequency current. Thus, the laser light output of the laser diode can be repeatedly turned on and off to maintain the laser diode in the multi-mode oscillation state.
In the art disclosed in Japanese Laid-Open Patent No. H9-178645 and U.S. Pat. No. 5,946,333, however, multi-mode oscillation state can not be maintained and mode hopping noise may be generated as the laser diode deteriorates over time.
In consideration of this problem, an object of the present invention is to provide a sample analyzer and sample analyzing method capable of automatically stabilizing the laser diode in multi-mode oscillation.